Sound absorbers



June 18, 1963 D. D'EUSTACHIO SOUND ABSORBERS 3 Sheets-Sheet 1 Filed Dec. 13, 1962 llh June 18, 1963 DEUSTACHIQ 3,094,188

SOUND ABSORBERS Filed Dec. 13, 1962 3 Sheets-Sheet 2 FIG. 5

United States Patent 3,094,188 SOUND ABSURBERS Dominic DEustachio, Pittsburgh, Pa., assignor to Pittsburgh Corning Corporation, Pittsburgh, Pa., 21 corporation of Pennsylvania Filed Dec. 13, 1962, Ser. No. 244,419 9 Claims. (Cl. 181-33) This invention relates to sound absorbers and more particularly to such absorbers which are suitable for mounting on a wall or other supporting surface.

This application is a continuation-in-part of my copending application Serial No. 743,425 filed June 20, 1958, which application is a continuation-in-part of my applications Serial No. 566,159 filed February 17, 1956, now Patent No. 2,853,147, and Serial No. 559,191 filed January 16, 1956, now abandoned.

One object of this invention is to provide a new and improved sound absorbing structure which is particularly well suited for mounting on a supporting surface, such as a wall, ceiling, the interior of a duct, etc.

Another object of the invention is to provide an improved sound absorber, particularly one having unusually high absorption of acoustic energy at low frequencies.

A further object of the invention is to provide a highly effective sound absorber having an acoustic impedance which is adjusted to suit the conditions under which the absorber is to function.

Still another object of the invention is to provide a sound absorbing structure for attaining a high distributed absorption over a large wall area.

It is well known that sound absorber-s commonly used heretofore were made of porous materials and often were provided with recesses extending into the material from the front face, that is, the face which was exposed to the sound. In some particularly effective arrangements, the opposite or back face of the material included an enlarged resonant chamber which communicated with the front face through a series of holes of a specific size and number. Representative absorbers of this latter type are more fully described in my Patent No. 2,703,627 granted March 8, 1955.

Sound absorbers employed heretofore have exhibited certain disadvantages, and these disadvantages have been of special moment in the absorption of low frequencies. Particularly in cases in which the absorbers were mounted directly on a wall or other supporting surface, the acoustic designer has not had the requisite degree of freedom in obtaining the optimum sound absorbing properties for the conditions under which the absorber is to function.

In certain advantageous embodiments of this invention, there is provided a sound absorbing structure comprising a body of porous material which contains non-intersecting recesses extending into it not only from the front face but also from the back face. The shape, size, depth and spacing of the recesses are chosen so as to obtain an acoustic impedance commensurates with the optimum impedance for a given installation.

In one of the embodiments of the invention, the sound absorber comprises a body of locally-brittle porous material, for example, open-celled frangible cellular glass, the said body being modified to improve its sound-absorbing properties by adjustment of its acoustic impedance, as described herein. The open-celled cellular glass preferably is obtained by subjecting closed-celled cellular glass to a high fluid pressure, as more fully described in my Patent No. 2,596,659 granted May 13, 1952. One advantage of the use of open-celled cellular glass is that the formation of the recesses is greatly facilitated and illustratively may be accomplished by a simple dieapunching operation, or by drilling, sawing, molding, etc.

3,094,188 Patented June 18, 1963 Certain important embodiments of sound absorbing structures in accordance with the invention employ soundabsorbing blocks of porous material which are applied directly to a substantially non-sound absorbing surface, such as the wall or ceiling of a room, or as a lining for the interior of air ducts and the like. Each block includes a first group of recesses extending into it from the front face and a second group of recesses extending from the back face, the front face being exposed to the sound to be absorbed and the back face being against the wall. The recesses from the front face are in close but non-intersecting proximity with those from the back face, so that coupling of acoustic energy is effected through the cells of the material therebetween.

Other features, objects and advantages will appear from the following more detailed description of illustrative embodiments of the invention, which will now be given in conjunction with the accompanying drawings.

In the drawings:

FIG. 1 is a front elevational view of a sound absorber in accordance with one illustrative embodiment of the invention, as seen from one of its principal faces;

FIG. 2 is an enlarged view of a portion of the absorber shown in FIG. 1, from the same viewpoint;

FIG. 3 is a cross-sectional view, partially broken away, taken along the line 33 in FIG. 2;

FIG. 4 is an enlarged fragmentary cross-sectional view, taken along the line 4-4 in FIG. 2, which is illustrative of the open-celled, cellular structure of absorbers constructed in accordance with the various illustrated embodiments of the invention;

FIG. 5 is an elevational view of an area of wall absorbers, applied, in spaced-apart relation, to a wall;

FIG. 6 is an enlarged cross-sectional view, partially broken away, taken along the line 66 in FIG. 5;

FIG. 7 is a front elevational view of a sound absorber in accordance with another illustrative embodiment of the invention, as seen from one of its principal faces;

FIG. 8 is a cross-sectional view taken along the line 88 in FIG. 7; and

FIG. 9 is a graph of absorption coefficient versus frequency showing representative performance characteristics of the absorber illustrated in FIGS. 1-4, as compared to corresponding characteristics of certain prior absorbers.

In FIGURES 1-4 there is shown a sound absorber comprising a slab 10 of open-celled cellular glass, having a series of elongated recesses 12 and 14 therein. 'Ihese recesses extend in from both the principal faces; thus, the recesses 112 extend into the slab from the front face 16, and the recesses 14 extend into the slab from the back or mounting face 17. The front face 16 is exposed to the sound to be absorbed, while the back face 17 is closed to the sound, such as by mounting the slab in contact with a wall or other supporting surface, for example, or by a layer 17a of pain"; or other separate closure. The recesses 12 extend inwardly into the intermediate portion of the absorber material and are close to and overlap or interleave with the recesses 14, as shown in FIGS. 3 and 4. It is preferable, however, that the recesses 12 not meet the recesses 14. Thus, all of the recesses in the slab are in spaced-apart, non-intersecting relationship with each other, so that the adjacent recesses from opposite faces are separated from one another throughout their full extents within the absorber body by portions of the body material.

Because the cells 13 (FIG. 4) of the material are open and interconnected by passages, such as the passages 15, each of the recesses 12 is in communication with its neighboring recesses 12, and is also in communication with its neighboring recesses 14. g

The volume of a typical recess 12 or 14 is large compared to the volume of a typical cell in the material. In diameter, a recess may, in some instances, be about the same size as the diameter of a typical cell. Each of these recesses is, however, in both its diameter and its length, of small dimension compared to the wavelengths of the acoustic energy which it is principally designed to absorb. In general, the absorption of the higher frequency components is not particularly diflicult with porous material. It is the absorption of the lower frequency components which is more difficult, and it is to improve the absorption of these low frequency components that the recesses are principally designed.

In FIGS. and 6 there is shown a plurality of the absorbers 10 applied to a rigid, flat surface 26, such as the wall or ceiling of a room or other surface having substantially non-sound absorbing characteristics. Each absorber has its front face 16 exposed to the sound to be absorbed and its back face 17 in contact with the surface 20. The recesses 12 in the front face communicate directly with the sound, while the recesses 14 in the back face communicate with the sound only through the recesses 12 and the intervening open cells of the absorber material.

In addition to being used on a non-sound absorbing surface individually or in abutting groups, the absorbers 10 are well adapted for use in a spaced-apart array, in accordance with the teachings of my application Serial No. 559,191, filed January 16, 1956. Considerable advantage has been found in employing arrays of spacedapart absorbers, wherein the absorbers have very high absorption, and "are of such dimensions that the width is in the range of from 6 to 24 inches and the length range is from 12 to 36 inches (these dimensions being of the order of a half-wavelength of some of the principal sound frequency components to be absorbed), and are spaced apart so that the total area of the wall itself, between the absorbers, is not less than one-half and not more than six times the total area of the exposed principal faces of the absorbers.

In the embodiments of FIGS. 1-6, the recesses 12 and 14 are shown as elongated round holes which extend in directions perpendicular to the absorber faces 16 and 17. In other good arrangements, the shape, depth, orientation or spacing of the recesses are varied to produce substantially any desired acoustic effect.

FIGURES 7 and 8 are illustrative of a wall absorber 26 in accordance with one of these latter arrangements. The absorber is fabricated from a slab of open-celled cellular glass and includes recesses in the form of parallel elongated slots 28 and 30 which are sawed or otherwise cut into each of the principal faces of the slab. The slots 28 extend inwardly from the front face 32 exposed to the sound, while the slots 36 similarly extend from the back face 34 in contact with the wall. The slots 28 and 30 extend part way through the slab, and overlap or interleave in the manner shown. The adjacent slots from opposite faces are separated from one another throughout their full extents within the absorber body by portions of the body material, so that fluctuations of air pressure within the slots 28, acting through the opencelled cellular material, affect or interact with fluctuations of air pressure within other slots 23, and also within neighboring slots 30, and vice versa.

In FIG. 8 the shape of the slots 28 and 30 in longitudinal section, may be seen. It may be noted in the drawing that the slots do not extend all the way to the edges of the slab, but terminate prior to the edges. Thus the slots are closed at each end. Near their ends, as may be seen in FIG. 8, the bottoms of the slots curve toward the surface of the slab.

In the various illustrated embodiments of the invention, the provision of recesses from the back or mounting face, in addition to those from the front face, provides substantially improved absorption and greatly increased flexibility in the design of a particular installation. This arises from the unexpected and little-understood fact that the recesses entering the absorber from its back face act as a series of individual resonant chambers and alter the acoustic impedance of the absorber, thus enabling the effective absorption of sound particularly in the lower frequencies.

An example of the effectiveness of the addition of the recesses from the back face is shown in FIG. 9. The data shown was taken using a sample of open-celled cellular glass arranged in an impedance tube. Measurements were made of the normal incidence absorption coefficient of the sample for frequencies from slightly below 200 cycles per second (c.p.s.) to 2000 cycles per second. The sample had a thickness of two inches and a diameter of four inches.

Curve A shows the normal incidence adsorption coefficient of the sample before any recesses were formed therein, either front or back. Curve B shows the absorption coefficient of the same sample after the formation of recesses from the front face only. Curve C shows the absorption coefficient of the same sample after the formation of additional recesses from the back face. The recesses from the front face of the sample were spaced three-eighths of an inch apart and were arranged in a manner similar to the recesses 12 (FIG. 3). The recesses from the back face were similarly spaced and were arranged in a manner similar to the recesses 14. These latter recesses were off-set three-sixteenths of an inch with respect to the recesses from the front face so that the recesses did not intersect. Each recess had a diameter of one-sixteenth of an inch and was one inch deep.

The sample for curve A showed an absorption coefficient which did not exceed about .38 througdiout the measured frequency range. The sample for curve B had an absorption coefficient which varied widely but which peaked at .70 at a frequency of 1000 cycles per second. The sample for curve C, which had recesses from both the front and back faces as shown in FIG. 3, had a maximum absorption coehici-ent of slightly over .80. Of even more importance is the fact that this coefficient was reached at a substantially lower frequency (400 cycles per second) than the maximum for curve B. The improved absorption resulting from the use of recesses from both faces of the absorber, as opposed to recesses from the front face only, is indicated by the shaded area D in FIG. 9.

In certain of its embodiments, the present invention may also be applied to acoustic filters or sound absorbers for air ducts. It is particularly useful, for this purpose, as a sound absorbing duct lining which may include special geometry to minimize hydrodynamic pressure drop while maintaining good sound attenuation.

It will be understood that one characteristic of the open-celled cellular glass described herein is that the opened cells are, to a great extent, in communication with one another, giving the material a porous property.

Although open-celled cellular glass is referred to herein as a preferred material, and it has a number of unique advantages, the invention is not, in its broadest aspect, limited to the use of this material, and in some cases, there may be employed other suitable porous materials.

While suitable illustrative embodiments of sound absorbing structures have been shown and described, along with certain modifications, it will be understood that various changes may be made without departing from the general principles and scope of the invention.

I claim:

1. A sound absorber, comprising a body of porous material having a front face exposed to the sound to be absorbed and a back face, said body including a first group of recesses entering the same from said front face and extending into the intermediate portion of said material, and including a second group of recesses entering the same from said back face, at least some of the adjacent recesses from opposite faces being in overlapping relationship with each other and being separated from one another throughout their full extents within said absorber body by portions of the body material, and covering means in contact With said back face for closing said second group of recesses, to prevent the transmission of sound energy thereto except through the body of material.

2. A sound absorber, comprising a body of porous material having a front face exposed to the sound to be absorbed and a back face, said body including a first group of recesses entering the same from said front face and extending into the intermediate portion of said material, and including a second group of recesses entering the same from said back face, at least some of the adjacent recesses from opposite faces being in overlapping relationship with each other and all of the adjacent recesses from opposite faces being separated from one another throughout their full extents within said absorber body by portions of the body material, and covering means in contact with said back face for closing said second group of recesses, to prevent the transmission of sound energy thereto except through the body of material.

3. A sound absorber of the character set forth in claim 2, in which the sum of the depths of each recess entering from said front face and an adjacent recess entering from said back face is greater than the thickness of said material.

4. A sound absorber, comprising a body of porous material having a front face exposed to the sound to be absorbed and a back face, said body including a first group of recesses entering the same from said front face and extending into the intermediate portion of the material, and including a second group of recesses entering .the same from said back face, each of the recesses from said front face being in overlapping relationship with the adjacent recesses from said back face and all of the adjacent recesses from opposite faces being separated from one another throughout their full extents within said absorber body by portions of the body material, and covering means in contact with said back face for closing said second group of recesses, to prevent the transmission of sound energy thereto except through the body of material.

5. A sound absorber, comprising a body of porous material having a front face exposed to the sound to be absorbed and a back face, said body including a plurality of elongated recesses extending into said faces in directions substantially perpendicular thereto, a first group of said recesses entering said body from said front face and extending into the intermediate portion of the material, and a second group of said recesses entering said body from said back face, at least some of the adjacent recesses from opposite faces being in overlapping relationship With each other and being separated from one another throughout their full extents within said absorber body by portions of the body material, and covering means in contact with said back face for closing said second group of recesses, to prevent the transmission of sound energy thereto except through the body of material.

6. A sound absorber, comprising a body of porous material having a front face exposed to the sound to be absorbed and a back face, said body including a plurality of parallel slots therein, a first group of said slots entering said body from said front face and extending into the intermediate portion of the material, and a second group of said slots entering said body from said back face, at least some of the adjacent slots from opposite faces being in overlapping relationship with each other and being separated from one another throughout their full extents within said absorber body by portions of the body material, and covering means in contact with said back face for closing said second group of recesses, to prevent the transmission of sound energy thereto except through the body of material.

7. A sound absorber, comprising a body of opencelled frangible cellular material having a front face exposed to the sound to be absorbed and a back face, said body including a first group of recesses entering the same from said front face and extending into the intermediate portion of said material, and including a second group of recesses entering the same from said back face, at least some of the adjacent recesses from opposite faces being in overlapping relationship With each other and being separated from one another throughout their full extents within said absorber body by portions of the body material, and covering means in contact with said back face for closing said second group of recesses, to prevent the transmission of sound energy thereto except through the body of material.

8. A sound absorber for mounting on a supporting wall, comprising a sound absorbing body of porous material having a front face exposed to the sound to be absorbed and a back face in contact with said wall, said body including a first group of recesses entering the same from said front face and extending into the intermediate portion of said material, and including a second group of recesses entering the same from said back face, at least some of the adjacent recesses from opposite faces being in overlapping relationship with each other and being separated from one another throughout their full extents within said absorber body by portions of the body material, said second group of recesses being closed by said wall, whereby sound energy is transmitted to said second group of recesses only through the pores of said material.

9. A sound absorber for mounting on a supporting wall, comprising a block of open-celled frangible cellular material having a front face exposed to the sound to be absorbed and a back face in contact with said wall, said block including a first group of recesses entering the same from said front face and extending into the intermediate portion of said material, and including a second group of recesses entering the same from said back face, each of the recesses from said front face being in overlapping relationship with the adjacent recesses from said back face and all of the adjacent recesses from opposite faces being separated from oneanother throughout their full extents Within said block by portions of the block material, said second group of recesses being closed by said Wall, whereby sound energy is transmitted to said second group of recesses only through the open cells of said material.

References Cited in the file of this patent UNITED STATES PATENTS 2,114,546 Slayter Apr. 19, 1938 2,127,867 Harvey Aug. 23, 1938 2,165,101 Hudson July 4, 1939 2,671,522 Bourgeois Mar. 9, 1954 2,703,627 DEustachio Mar. 8, 1955 2,856,323 Gordon Oct. 14, 1958 

1. A SOUND ABSORBER, COMPRISING A BODY OF POROUS MATERIAL HAVING A FRONT FACE EXPOSED TO THE SOUND TO BE ABSORBED AND A BACK FACE, SAID BODY INCLUDING A FIRST GROUP OF RECESSES ENTERING THE SAME FROM SAID FRONT FACE AND EXTENDING INTO THE INTERMEDIATE PORTION OF SAID MATERIAL, AND INCLUDING A SECOND GROUP OF RECESSES ENTERING THE SAME FROM SAID BACK FACE, AT LEAST SOME OF THE ADJACENT RECESSES FROM OPPOSITE FACES BEING IN OVERLAPPING RELATIONSHIP WITH EACH OTHER AND BEING SEPARATED FROM ONE ANOTHER THROUGHOUT THEIR FULL EXTENDS WITHIN SAID ABSORBER BODY BY PORTIONS OF THE BODY MATERIAL, AND COVERING MEANS IN CONTACT WITH SAID BACK FACE FOR CLOSING SAID SECOND GROUP OF RECESSES, TO PREVENT THE TRANSMISSION OF SOUND ENERGY THERETO EXCEPT THROUGH THE BODY OF MATERIAL. 